1. Field of the Invention
This invention relates in general to a method and apparatus for measuring wafer resistance of a semiconductor wafer, and more particularly to a contactless approach for automatically measuring wafer resistance.
2. Related Art
The wafer or sheet resistance (and resistivity) of doped silicon wafers has long been used as a primary measurement to determine the doping characteristic of silicon wafers. The xe2x80x9cFour Point Probexe2x80x9d method has been the standard means of determining the sheet resistance. The xe2x80x9cFour Point Probexe2x80x9d method is a contact measurement that uses precisely spaced, spring loaded, pointed tip probes to inject DC current into the surface of a silicon wafer and measure the DC voltage drop across a specific distance on the wafer surface. The four probe tips are arranged in a linear array and are typically spaced 1.59 mm (0.0625 inch) apart. The sheet resistance measurement is performed by:
Pressing the probe tips on to the wafer surface,
Injecting a known value, constant DC current into the wafer material through the outer two probe tips,
Sensing the DC voltage across the two inner probe tips.
The xe2x80x9cFour Point Probexe2x80x9d sheet resistance measurement has the following disadvantages:
The probe tips scratch the surface of the silicon wafer.
Making a low resistance contact is sometimes difficult because of surface oxide layers.
Correction factors have to be included in the sheet resistance measurement process to compensate for the xe2x80x9cFour Point Probexe2x80x9d tip spacing and wafer thickness.
Standard wafers are required for the calibration process. Standard wafers are expensive and delicate.
Sheet resistivity measurements have also been performed using a non-contact, Eddy Current Gauge system. An inductive sensor in the eddy-current gauge generates a high frequency, AC magnetic field that is directed at the silicon wafer surface. The sinusoidal magnetic field causes eddy-currents to circulate in the wafer material and produce a power loss in the wafer material resistance. The power transmitted to the silicon material by the sensor is detected by the eddy-current gauge electronic circuitry and is used as measurement of the wafer sheet resistivity. No physical contact between the eddy-current sensor and the wafer material is required because all the power is transmitted through the magnetic field. The eddy-current, sheet resistance measurement approach has the following disadvantages:
The accuracy of the resistivity measurement results is dependent on the distance between the sensor and the wafer surface.
The calibration of the eddy-current sensor is dependent on the frequency of the sine wave signal used to generate the AC magnetic field in the wafer.
Calibration of the system has to be performed by standard wafers, which are expensive and delicate.
The operational measurement range of specific sensors is limited to resistance valued under about 5 Ohms because of the resistance of the sensor coil used to produce the magnetic field that generates the eddy-currents.
Shortcomings of the prior art are overcome and additional benefits realized, in accordance with the principles of the present invention, through the provision of apparatus for non-contact determination of wafer resistance. The apparatus includes a first sensor element, separated from a surface of said wafer by a first air gap, for capacitively coupling an AC drive signal into a portion of said wafer. A second sensor element, separated from the surface of said wafer by a second air gap, capacitively couples an AC output signal out of said wafer portion. An inductor is in series with at least one of said first sensor element and the second sensor element. The apparatus also includes means for automatically tuning a frequency of the AC drive signal to a resonant frequency at which capacitance impedance of the first air gap and of the second air gap is canceled by inductive impedance of the inductor.
In another aspect, the present invention provides apparatus for non-contact determination of wafer resistance of a semiconductor wafer. This apparatus includes a sensor element spaced from the wafer; first means for providing an AC drive signal to said sensor element for producing an AC current in a current path extending through a portion of the wafer; second means for automatically setting a frequency of the drive signal to substantially cancel impedance, other than wafer resistance, along said current path; and third means for automatically determining a voltage value of the AC drive signal required to drive an AC current signal of fixed magnitude through the wafer portion. The voltage value provides a measure of the wafer resistance.
In a further aspect, the present invention includes a method for contactless measurement of wafer resistance of a semiconductor wafer. The method involves applying an AC drive signal to a sensor element spaced from the wafer to produce an AC current signal in a current path extending through a portion of the wafer. A frequency of the drive signal is automatically set to substantially cancel impedance, other than the wafer resistance, along the current path. A voltage value of the drive signal required to drive an AC current signal of fixed magnitude through the wafer portion is automatically determined. The voltage value provides a measure of wafer resistance.